Vacuum panels, particularly those made with plastic materials, are becoming more prevalent for use in many different fields wherein thermal insulation at temperatures lower than about 100° C. is required.
Examples of such vacuum panels include walls of domestic and industrial refrigerators, drinks dispensing machines (wherein thermal insulation is required above all in order to separate the portion of the hot drinks, generally at about 70. degree. C., from that of the cold drinks), or the containers for isothermal transportation. Moreover, applications of these panels are noticeable in the building field or in the car industry. These panels can be used for example to produce a prefabricated insulating wall, by inserting inside said wall an adequate number of panels and by placing them side by side.
A vacuum panel is generally formed of an envelope inside which a filling material is present. The envelope has the function of preventing or reducing as much as possible the entrance of atmospheric gases inside the panel, so as to keep a vacuum level compatible with the thermal insulation degree required by the application. To this purpose, the envelope is made with so-called “barrier” sheets, characterized by a gas permeability being as low as possible, which can be formed of a single component but more frequently are multi-layers. In this last case the “barrier” effect is conferred by one of the component layers, whereas the other layers generally have functions of mechanical support and protection of the barrier layer.
The filling material has also the function of spacing apart the two opposite faces of the envelope when vacuum is created in the panel. During panel evacuation, in fact, the envelope adheres to the filling material, because of the difference between the external atmospheric pressure and the reduced internal pressure of the panel. Said filling material can be inorganic, such as silica powder, glass fibers, aerogels, diatomaceous earth, etc., or organic, such as rigid foams of polyurethane or polystyrene, both in the form of boards and of powders. The filling material must be anyway porous or discontinuous, so that the pores or the interstices can be evacuated.
In order to obtain a good insulation with these panels, it is necessary that the internal pressure remains constant or, in any case, lower than a maximum prefixed value, during the whole life of the panel, or most of it; if this does not occur the insulating capability decreases and, at the same time, the thermal flow from outside towards the inside of the device on which the panel is mounted increases, with consequences on the working of the device itself.
Since the permeation of traces of atmospheric gases into the panel is practically unavoidable, these panels contain in most cases also one or more getter materials capable of sorbing these gases so as to maintain the pressure inside the panel at the desired values. Preferred is the use of systems with two or three getter materials, containing at least a chemical moisture sorber and at least one component chosen among an oxide of a transition metal (having the main function of sorbing hydrogen, CO and hydrocarbons) and an alloy based on barium and lithium (having the main function of sorbing nitrogen). Various getter systems of this kind are sold from the applicant under the name COMBOGETTER®, among which in particular systems containing a moisture sorber and powder of an alloy based on barium and lithium, disclosed in the patent EP-B-769117; and getter systems containing a moisture sorber and an oxide of a transition metal, with the optional addition of powder of an alloy based on barium and lithium, disclosed in patent EP-B-757920.
Irrespective of the use or not of getter materials inside the panel, the users of vacuum panels in final applications have the need of measuring and checking that the internal pressure is the desired one and as declared from producer, and possibly that this pressure is kept for a long period.
Patent JP-A-5-142075 discloses a method for estimating the internal pressure of panels, consisting in inserting into an evacuation chamber an insulating panel, having a recess on its external surface; when the pressure inside the chamber is equal to the internal pressure of the panel, the external envelope raises. On the base of the particularly evident displacement of the part of envelope covering the recess, it is possible to determine the internal pressure of the vacuum panel.
Evacuation chambers are produced also by the company NPC America Corporation, and, as illustrated in specific commercial bulletins, laser sensors are used in these for measuring the movement of the envelope of the panels of which it is desirable to determine the pressure. Since the chamber used for this measurement must be capable of measuring the internal pressure of panels in a wide range of sizes, said chamber may be cumbersome, as well as expensive. Besides, the evacuation of this chamber can take rather long times, as well as the loading phase of the panel inside the chamber itself, since the panel must be arranged carefully and in a reproducible way, in order to meet the geometrical constraints imposed by the optical path of the laser beam. Furthermore, the evacuation chamber cannot be easily moved, because of its weight, and therefore the panels must be moved in an area suitably chosen for the working of the chamber itself.
Hence, there is in the technique a need for a system of small dimensions, cheap, and easy to be handled and moved, which allows the internal pressure of a vacuum panel to be measured quickly, without the necessity to insert said panel completely inside an evacuation chamber.